The invention relates to a measuring device for determining the oxygen activity in metal melts, especially iron, steel or cast iron melts, or slag melts, with a measuring head arranged on the end of a carrier tube, on which an electrochemical measuring cell is arranged, wherein the electrochemical measuring cell has solid electrolyte tube closed on one end, which is surrounded on the closed end and at least on a portion of its periphery by a steel tube closed on one end, wherein a reference material and a filler material adjoining the reference material are arranged inside the solid electrolyte tube on its closed end, wherein a metal rod is arranged as an electrode along the solid electrolyte tube in such a manner that, on the one hand, it stands in contact with the reference material and, on the other hand, it projects out of the open end of the solid electrolyte tube, and wherein the open end of the solid electrolyte tube has a closure.
A measuring device of this type is known from German published patent application DE 30 21 949 A1. This measuring device has a solid electrolyte tube, which is closed gas-tight on its open end by a rubber stopper. The rubber stopper extends into the solid electrolyte tube and is pressed against its inner wall, whereby the degree of sealing depends, among other things, on the size of the contact surface on the solid electrolyte tube. During the measuring process, this device is dipped into a steel melt. There then follows a very rapid and extreme temperature change. Due to the gases confined in the interior of the solid electrolyte tube, the internal pressure is thereby considerably increased, so that particular requirements must be placed on the strength of the solid electrolyte tube or its metal casing.
To solve a similar problem with a quartz glass tube, it is proposed in British patent specification GB 1 473 761 to guide gases from an inert material outside through a vent tube. A further measuring device for determining the oxygen activity of metal melts is described in German published patent application DE 28 42 136 A1.